Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EST Mon Jan 06 2020

Valid 12Z Mon Jan 06 2020 - 12Z Thu Jan 09 2020


Days 1-3...

...Pacific Northwest to the Northern Rockies...
An active period in the Northwest continues as the next in a
series of shortwave troughs and associated surface fronts moves
inland today. The first feature is a warm front lifting onshore
the PacNW this aftn, which will spread moisture well inland on
Pacific jet energy, but also raise snow levels to 4000-5000 ft in
WA, lower further east. The subsequent warm front and another
shortwave will move onshore Tuesday, bringing continued
precipitation but with snow levels crashing back down to 1000-2000
ft. This collapse of the snow levels will persist on D3, falling
below 500 ft as far south as Wyoming, but precipitation will be
less significant and widespread by Wednesday.

Prolonged moisture advection combined with ascent through height
falls, surface fronts, and jet level diffluence will produce heavy
snow across much of the high terrain from the Olympics in WA, the
Cascades of WA/OR, and eastward into the Northern Rockies and
other ranges of ID/MT/WY. The heaviest snowfall is likely in WA,
and on Day 1, when WPC probabilities for 12 inches are high in the
Olympics and Cascades. Despite somewhat lowered SLRs due to likely
riming, isolated amounts to 4 ft are possible in the highest
terrain. Further east into ID/MT, WPC probabilities are also high
for 12 inches, and amounts to 2 ft are possible.

A secondary surge of moisture Tuesday will continue the likelihood
for heavy snowfall, with the lowering snow levels producing more
widespread coverage of accumulations. The heaviest snowfall is
again likely in the Cascades, in WA and OR Tuesday, as WPC
probabilities are moderate to high for 12 inches, with locally
much higher amounts possible. By day 3 the forcing for ascent and
available PWATs are lower, but snow will likely continue in WA/OR,
while also spreading southeast into the terrain of the Great Basin
and becoming more widespread in WY/MT/ID due to the lowering snow
levels. WPC probabilities for 4 inches on day 3 cover a large area
above 1000 ft from northern CA into the Wasatch of UT, and points
north.


...Great Lakes...
A series of two weak surface lows will track just north of the
Great Lakes. The first of this will occur today, the second
Tuesday night into Wednesday. While these will be weak and
contribute little moisture or forcing to the region, CAA behind
each one across the Lakes will produce LES in the favorable W/NW
snow belts downwind of the lakes. On day 1 /Monday/, WPC
probabilities are high for 6 inches int he Tug Hill Plateau east
of Lake Ontario. Tuesday/Wednesday, the CAA is more favorable for
larger spatial coverage of LES, and WPC probabilities for 4 inches
are moderate to high in the U.P. of MI on D2, and increase SE of
Lake Erie, as well as Lake Ontario again, on D3.
 

...Central Appalachians to the Northeast...
A potent southern stream shortwave will produce surface
cyclogenesis lifting up through the Central Appalachians and off
the Mid-Atlantic coast Tuesday before racing quickly east of the
Gulf of Maine Wednesday. Although the system is quick moving and
small, robust forcing through potent WAA/fgen should overcome the
marginal thermal profiles to produce light to moderate snowfall
from the Mountains of NC, through the I-95 corridor of the
Mid-Atlantic, and into eastern New England. While snowfall may be
rate dependent due to the warm near-surface layer, profiles
indicate the strongest ascent may collocate with the DGZ, and
snowfall potential has increased overnight. The forcing may offset
from the surface low east of I-95, but the strengthening of the
surface low may produce another swath of moderate snow in eastern
New England. This leaves two regions of highest probability for
snowfall.

On day 2, WPC probabilities for 4 inches are confined to the
terrain of WV, but probabilities for 1" spread well inland towards
I-95 in VA/MD, and the evening commute Tuesday in the urban
corridor could be affected by moderate snow rates. Later D2 into
D3, as the low deepens and moves east offshore, a modest
deformation band may try to pivot snowfall back into
NJ/LI/southern New England, with mid-level troughing potentially
leading to lingering snowfall into ME as well. WPC probabilities
are low for 4 inches outside of eastern Maine.

The probability of significant icing (0.25 inch or more) is less
than 10 percent.

Weiss